1. Field of the Invention
The present invention relates to an interaxle differential lockout shift unit for a tandem drive axle assembly, and more particularly, relates to a pneumatically actuated interaxle differential lockout shift unit which includes at least one piston chamber which is vented internally of the drive axle assembly housing.
2. Description of the Prior Art
Tandem drive axle systems for vehicles, such as heavy duty trucks, are well known in the prior art. The essence of a tandem drive axle assembly is its ability to drive more than one drive axle, generally two or three rear drive axles. This is typically accomplished by means of a power (or torque) divider, usually a differential, in which one of the differential side gears transmits a certain percentage of the input torque to the first drive axle while the other differential side gear transmits the remaining torque by means of an output shaft to a subsequent axle assembly including a second drive axle. A tandem axle assembly of this well known in the art type is illustrated in U.S. Pat. No. 3,000,456, assigned to the assignee of the present invention and incorporated herein by reference.
The power divider differential, usually referred to as an "interaxle differential", is usually provided with a selectively engagable and disengagable interaxle differential lockout clutch and a lockout shift unit for selectively engaging and disengaging the clutch. Typically the lockout clutch is jaw clutch and the shift unit is pneumatically actuated from a spring biased first position to an pneumatically maintained second position.
Tandem axle power dividers of this type are well known in the prior art as may be seen by reference to U.S. Pat. Nos. 3,916,728; 3,929,200 and 4,050,534, all assigned to the assignee of this invention and the disclosures of which are all incorporated herein by reference.
While the prior art tandem drive axle systems, and the interaxle differential lockout shift units used therewith, are widely used and very commercially successful, they are not totally satisfactory as the shift units, usually mounted on the front-rear axle housing, are vented to atmosphere and thus prone to blockage from airborne contamination which can prevent or inhibit the shift units from properly functioning. Further, venting and drawing from atmosphere at the exterior of an axle housing allows corrosive matter, such as road salt and the like, to be drawn into the shift unit.